Plotter-type graphics systems generally include a plotter head bearing a writing instrument that is displaceable on a print medium to trace graphic patterns thereon. In the so-called horizontal plotters, the print medium is fixed and the writing head is displaceable over the medium along two rectangular axes. In other plotters, the print medium is moveable in an X-direction and the writing head is moveable in a Y-direction, perpendicular to the X-direction. The writing instrument is further displaceable in a Z direction, perpendicular to the print medium, between a work position contacting the print medium and a position displaced from the medium. The writing instrument used is selected among a set of writing instruments stored in a storage device, such as a barrel for example. Several writing instruments, e.g. having different colours or thicknesses, can thus be used for a same drawing.
It is important to know at any moment which writing instrument is mounted on the head. On the one hand it is required to modify such plotting parameters as speed, acceleration or applied pressure, depending on the type of writing instrument used, e.g. whether it is a fibre-tip, ballpoint or tube-point; on the other hand, it may be useful to check whether the writing instrument carried on the head corresponds to the one intended to be used.
A known identification procedure consists in picking out the different writing instruments in terms of the positions they occupy in the storage device, the instrument type being stored in a memory within the machine by an operator when the different instruments are loaded into the storage device. Another known identification procedure consists in placing optical references on the writing instruments, such as reflecting annular strips whose width and location depend on the type of instrument. The type of instrument picked by the plotter head is then determined by detecting the width and location of these strips by means of an optical transmitter-receiver. This system has the drawback of being sensitive to ambient light and to dust that can darken the reflecting strips. The reflecting strips can be damaged when changing instruments, and the system requires purpose-designed detection equipment that can be difficult to adjust. In other systems, identification of the instrument type is obtained by identification of the barrel, which prohibits having an assortment of several types of instrument in a common barrel.